COMMENT pubs.acs.org/Langmuir
Reply to Comment on Water Droplet Motion Control on Superhydrophobic Surfaces: Exploiting the Wenzel-to-Cassie Transition ’ WETTING TRANSITIONS ON HEATED HYDROPHOBIC SURFACES The comment of Bormashenko (Bormashenko, E. Langmuir 2011, 27, 12769 12770) lucidly describes how the contact angle of a droplet may increase due to evaporation at the three-phase line. We feel that the description offered does not significantly differ from the picture we attempted to paint in our original manuscript where we identified that the difference between the two superhydrophobic states is the presence of water between the pillars in the pinned state at the three-phase line as seen in Figure 41—but the argument as presented by Bormashenko (Bormashenko, E. Langmuir 2011, 27, 12769 12770) is more lucid and as such will add to the understanding of the phenomena. Indeed, as stated by Bormashenko we cannot be sure whether the low contact angle starting state for our droplet is the Wenzel or Cassie Impregnating state.2 To explore this further, we present an image we obtained of a droplet obtained at 190 °C (see Figure 1) where it can be seen
that some areas of the rough surface beneath the drop are not filled with liquid. That is, the droplet exists over a range of circumstances where nearly all the roughness is filled with liquid (Figure 1a) to that where only a small percentage of the roughness is filled (Figure 1b). We estimate from other images that with as little as 10% of the features under the footprint filled, the droplet is immobile and resists conversion to the Cassie state. We find that the change in contact angle occurs rapidly (over a period of ∼0.20 s, see Figure 2). A second possible explanation remains for these observations and as yet we cannot distinguish which mechanism is dominant. The change in contact angle observed may not be driven by evaporation but rather by changes in the interfacial energies. We discussed this in our paper1 and have called it a Sullivan like transition,3 as Sullivan described a transition driven by surface energy from partial wetting to nonwetting with increasing temperature for low energy surfaces, though this was an equilibrium process for a Lennard-Jones fluid. Indeed, transitions due to
Figure 1. A droplet evaporating on a superhydrophobic substrate heated to 190 °C. Note that, in some regions, indicated by the arrows the droplet is not impaled on the surface roughness. These regions form and disappear during the heating process. Frames C and D are magnified images of A and B. Received: September 15, 2011 Published: October 04, 2011 r 2011 American Chemical Society
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Langmuir
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Figure 2. Four sequential video frames (from left to right) captured at intervals of 0.06 s showing the rapid transition from the Wenzel to the Cassie state on a superhydrophobic surface at 190 °C. For comparison, the droplet took approximately 32 s to evaporate completely.
evaporation at the three-phase line or due to surface energetics are closely linked as the energy of the solid vapor interface is intimately related to any adsorbed film of liquid. If this liquid is removed by evaporation, the interfacial energy must change. We note that the phenomenon is very similar on flat smooth surfaces: here, a Cassie impregnating state cannot exist, but the phenomena may be the same in that evaporation of a prewetting film may drive the transition. On these surfaces, the high contact angle achieved does not remain upon cooling.4 Guangming Liu,† Andrei Rode,§ Lan Fu,# and Vincent S. J. Craig*,‡ †
Department of Chemical Physics, University of Science and Technology of China, Hefei National Laboratory for Physical Sciences at the Microscale, Hefei, Anhui, China, 230026
§
Laser Physics Centre, #Department of Electronic Materials Engineering, Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra, ACT 0200, Australia
‡
’ AUTHOR INFORMATION Corresponding Author
*
[email protected].
’ REFERENCES (1) Liu, G. M.; Fu, L.; Rode, A. V.; Craig, V. S. J. Langmuir 2011, 27 (6), 2595–2600. (2) (a) Bormashenko, E.; Pogreb, R.; Stein, T.; Whyman, G.; Erlich, M.; Musin, A.; Machavariani, V.; Aurbach, D. Phys. Chem. Chem. Phys. 2008, 10 (27), 4056–4061. (b) Feng, L.; Zhang, Y. A.; Xi, J. M.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Langmuir 2008, 24 (8), 4114–4119. (3) Sullivan, D. E. J. Chem. Phys. 1981, 74 (4), 2604–2615. (4) Liu, G. M.; Craig, V. S. J. Faraday Discuss. 2010, 146, 141–151.
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